1. Field of the Invention
The present invention relates to a control system for controlling speed changes in a continuously variable transmission and, more particularly, to a shift control system for controlling a continuously variable transmission to restrict an input speed.
2. Related Art
The continuously variable transmission can change a gear ratio continuously, as well known in the art, to set the gear ratio so that a speed of a prime mover such as an engine connected to the input side of the transmission may be suitable for the current running state. In the continuously variable transmission mounted on a vehicle employing an internal combustion engine (as will be abbreviated into xe2x80x9cenginexe2x80x9d) as a prime mover, therefore, the gear ratio is so controlled as to equalize a speed of the engine to a speed achieved at an optimum running point for the best fuel economy.
If such control is executed in all running states, however, the driving force may be short to deteriorate the drivability. This is exemplified by the case in which the vehicle runs on an uphill. If the continuously variable transmission is controlled to reduce the engine speed as the vehicle speed rises, the driving torque lowers. In Japanese Patent Laid-Open No. 7-71556, therefore, there is disclosed a control system for setting a target input speed of the continuously variable transmission to a larger value for a heavier road load of the uphill run. This control system is constructed such that a lower limit value of the target input speed is restricted when the road load is so heavy as experienced by the uphill run.
In the invention disclosed in the aforementioned Laid-Open, the gear ratio is so controlled, when the road load is heavy, as to increase the input speed of the continuously variable transmission, i.e., the engine speed. Therefore, the driving force is retained on the uphill to improve the drivability. However, the system of the prior art thus far described inhibits the setting of a target input speed on a lower speed side, as could otherwise be set for an ordinary run, when the road load is heavy, so that a restricted lower limit speed on a higher speed side is employed as the target input speed.
If an actual input speed at the instant when the decision of a heavy load state for the high road load holds is lower than the restricted lower limit value, therefore, the target input speed is set to a lower limit value on a higher speed side by deciding the heavy load state and restricting the lower limit speed accordingly. Although no accelerator depression is performed for augmenting the driving force, more specifically, the input speed of the continuously variable transmission, i.e., the engine speed may increase according to the decision of the heavy load state, and the driver may be given a physical discomfort, or the drivability may deteriorate.
In order to eliminate this disadvantage, it is conceivable to make the following restriction. There may occur the case in which the actual input speed at the time of deciding the heavy road load state is lower than the aforementioned lower limit value. In this case, the lower limit value of the target input speed is restricted after the actual input speed exceeds the lower limit value. In the system for this control, however, the actual input speed may not exceed the lower limit value although the decision of the heavy load state is made. In this case, the input speed is kept at a low value, so that the gear ratio set by the continuously variable transmission is kept at a value on the high side (or on the higher vehicle speed side). If a braking operation is made in this running state, therefore, the vehicle is stopped for a short time because of the heavy road load. The revolutions of the continuously variable transmission stop accordingly, and it may be impossible to set the gear ratio on the lowest side (on the lower vehicle speed side), as demanded at the starting time. In the belt-type continuously variable transmission, more specifically, it is known that a transmission member such as a belt is brought into frictional contact with a rotary element on the drive side and a rotary element on the output side so that the gear ratio may be changed by the changes in the contact positions. If the rotations of the continuously variable transmission stop, the contact positions of the transmission member cannot be changed. At the starting time, it is inevitable to employ a gear ratio smaller than that on the lowest speed side, so that the driving force for the start becomes short.
A main object of the invention is to provide a control system for a continuously variable transmission, which can make a driving force necessary and sufficient for a heavy load time and which has excellent characteristics in that a gear ratio for the vehicle to stop is reliably set to a gear ratio on the lowest speed side.
In order to achieve the above-specified object, the invention is characterized in that the restriction executing conditions, as based on the lower limit speed set for the target input speed, are changed according to the road load. More specifically, in a shift control system for a continuously variable transmission of the invention, road load on a vehicle is detected, a lower limit speed according to the detected road load is set, a target input speed according to the running state of a vehicle is restricted with the low limit speed, and gear ratio of the continuously variable transmission is controlled so that an actual input speed may be equal to the target input speed restricted. Moreover, the shift control system of this invention is characterized in that the target input speed is restricted on condition that the actual input speed exceeds the lower limit value, when the road load is light, and the target input speed is restricted irrespective of the condition when the road load is heavy.
In the invention, therefore, according to the road load of the vehicle, the lower limit value of the target input speed of the continuously variable transmission is set, but the target input speed on the basis of the lower limit value is restricted on the basis of the magnitude of the road load. When the road load at or after the time of setting the lower limit speed is light, more specifically, the target input speed on the basis of the lower limit value is restricted on condition that the actual input speed exceeds the lower limit value. When the road load is light or when the actual input speed is less than the lower limit value, therefore, the target input speed is not restricted immediately after the lower limit speed is set. While the vehicle is running on an uphill road with a gentle gradient as in the case in which the road load is light, therefore, the speed of the prime mover is prevented from being increased although no accelerator depression is performed. If the actual input speed exceeds the lower limit speed, on the other hand, the target input speed is restricted on the basis of the lower limit value, so that the input speed, i.e., the driving force speed can be kept later at a large value to retain the driving force. In short, the drivability is improved. If the road load is heavy at the time or after a lower limit speed is set according to the road load, on the other hand, the target input speed is immediately restricted to the lower limit value. As a result, a gear ratio of the continuously variable transmission is set to that on the low side (or the low vehicle speed side). Even when the vehicle is stopped for a short time by the decelerating or braking operation, therefore, the gear ratio can be returned to that of or approximate to the lowest speed side. Thus, according to the invention, it is possible to keep both the excellent drivability and the returning performance of the gear ratio to the lowest speed side.
According to the invention, on the other hand, the lower limit speed can be changed and set to a value according to the detected road load and a vehicle speed, and to a value higher than the road load value under the road load at least in a low vehicle speed range, when the road load is heavy.
With this construction, therefore, the lower limit speed is set on the basis of the road load and the vehicle speed. When the road load is heavy and the vehicle speed is in the low vehicle speed range, moreover, the lower limit speed is set to a value higher than the road load speed under the road load. On an uphill road with a steep gradient, as an example of the heavy load, therefore, there is kept the state in which the actual input speed is increased to a high value. Even if the vehicle is stopped for a short time in accordance with the braking operation, therefore, the gear ratio easily returns to that on the lowest speed side.
In addition to the aforementioned construction, the shift control system of the invention may be constructed to further comprise shift position selecting device for selecting a shift position for an ordinary run and a brake position for causing a braking action by a prime mover more than the case in which the vehicle runs at the shift position. The restriction on the target input speed can be released on condition that the road load is light, when a shifting operation is made from the brake position to the shift position for the ordinary run.
With the construction having the heavy road load, therefore, even when the vehicle is stopped for a short time by the deceleration or by the braking operation, the gear ratio easily returns to that on the lowest speed side for the short time because it is relatively high. With the construction having the light road load, on the other hand, when the shift is changed from the brake position to the shift position for the ordinary run, that is, when the upshift is manually effected, there is released the restriction on the target input speed based on the lower limit value. As a result, the target input speed is reduced to reduce the speed of the prime mover. This is the change in the gear ratio in accordance with the intention of the driver to change speed. In this invention, therefore, it is possible to make a speed change according to the intention of the driver without deteriorating the returnability of the gear ratio to the lowest speed side.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read with reference to the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustrations only and are not intended as a definition of the limits of the invention.